Municipal Effluent Treatment With GeoSIL

The properties of  GeoSIL which make it a highly attractive oxidant for sewage treatment are easy and safe handling, lack of reaction with ammoniacal products, selectivity particularly to sulphide and enhancement of secondary sewage oxidation processes.

The most important use of  GeoSIL in sewage treatment is the control of hydrogen sulphide in septic sewage and sludge. This is described below and also included in a review of the use of GeoSIL in activated sludge processes to control bulking, to provide a source of supplementary oxygen and to overcome denitrification in secondary settlement.

Sulphide Control

The second half of the 1970 - 80 decade saw considerably increased recognition, in most industrialised countries, of the problems associated with the formation of hydrogen sulphide in septic sewage.

When sewage has limited or no access to air, aerobic micro-organisms abstract oxygen from dissolved salts after depletion of free oxygen in solution. First to be attacked are nitrates, followed by sulphates, normally present in far greater quantities than nitrates, thus leading to the formation of nitrogen and hydrogen sulphide. This latter product, gaseous at atmospheric pressure, is nearly as toxic as hydrogen cyanide, has nauseating "rotten egg" odour and is biologically re-oxidised to sulphuric acid when released to the air, thus leading to corrosion of concrete, cast iron/mild steel and copper switchgear.

Undoubtedly the principal area where sulphide control is required is in pumped or rising mains. Nevertheless, sulphide problems also occur in lengthy, probably poorly ventilated, gravity sewers in large catchment areas. Both of these situations are amenable to treatment with GeoSIL .

Hydrogen sulphide frequently, and in many cases rapidly, develops in sewage sludge on storage. This is another area where the addition of  GeoSIL capable of irreversibly overcoming hydrogen sulphide evolution, is usually the preferred solution.

Principles

Hydrogen sulphide may be controlled in sewage by four principal mechanisms:

q  Preventative

The provision of preferential source of combined oxygen or the bactericidal inhibition of sulphate reduction.

q  Curative

Chemical oxidation of destruction of sulphide.

q  Suppressive

Conversion of hydrogen sulphide into a non-volatile form, e.g. precipitation or ionisation.

q  Palliative

Masking of hydrogen sulphide, neither preventing its formation nor destroying it.

Suppressive methods such as precipitation of iron sulphides with copperas or conversion of hydrogen sulphide to non-volatile sulphide ions with lime are incomplete in that hydrogen sulphide can be re-formed by changes of conditions. Palliative measures, namely the use of masking agents, give no reduction in toxicity and corrosion hazards.

The most effective control is by mechanisms a. and b. in that hydrogen sulphide is completely and irreversibly eliminated from the system. If GeoSIL is added before septic conditions arise it provides a preferential source of oxygen to microorganisms and avoids the reduction of sulphate to sulphide:

          2H2O2 = 2H2O + O2

Alternatively, under the near neutral pH conditions usually encountered in sewage, any hydrogen sulphide present is oxidised in 15 - 60 minutes to elemental or colloidal sulphur:

          H2S + H2O2 = 2H2O + S

Thus GeoSIL can both prevent hydrogen sulphide formation and easily destroy it when it is already present.